Not Applicable.
1. Field of the Invention
This invention relates to solid fuel ramjet engines and more particularly to a device for controlling/varying thrust. In more particularity the invention relates to a device for varying the heat transfer to the solid fuel by exciting pressure oscillations at a controlled amplitude in the combustor and consequently controlling the fuel regression rate and thrust level.
2. Description of the Related Art
Solid fuel ramjet engines, whether brought to operational speed by a booster engine or air dropped from a vehicle, depend upon the introduction of air into the engine due to its forward motion. Thus the term ramjet is used. As the ram air passes through a solid fuel grain within a combustor, fuel rich gases generated by the solid fuel react with oxygen in the air inside the solid fuel bore and in the further downstream located mixing chamber of the combustor and pass out of the engine via a nozzle producing thrust.
The rate at which the fuel rich gases are generated from the solid fuel grain, in general, depends on the fuel grain surface area, temperature and pressure within the combustor and the mass rate of air flow over the fuel grain. Due to a wide range of flight conditions encountered by the engine during operation, the air mass flow varies considerably while the missile is changing speed and altitude. Without some means of controlling the burn rate of the solid fuel in response to changes in air mass flow excessively rich combustion chamber conditions will exist, which is very wasteful of fuel and reduces the range of the vehicle. Additionally, engine variables, such as changes in the solid fuel grain area, thrust, and combustor temperatures and pressures, as well as missile flight parameters, such as Mach number and angle of attack necessitate changes in fuel burn rate to maintain the variable within acceptable limits.
In addition, solid fuel ramjets for tactical missile propulsion require thrust throttling capability to avoid overspeeding. Ramjets, in their standard design, have no means for on-demand thrust throttling. This results in overspeeding, because higher speeds increase the air flow velocity over the fuel grain, which results in an increase in the regression rate. The result is a continual increase in thrust, which adversely affects the control and maneuverability of the missile system and limits the range. Currently, the problem of thrust throttling is addressed by varying the air mass flow through the fuel grain using mechanical devices with moving parts.
Heretofore, efforts have been directed to engine configurations that either bypass a portion of the ram air around the solid fuel combustor or vent a portion of the air outside of the vehicle. In both cases the object is to reduce the air flow into the solid fuel combustor and reduce the burn rate of the fuel.
Another common technique used to control the uniformity, but not directly the rate of fuel burning, includes a tube-in-hole arrangement, whereby a tube is inserted into the fuel grain inlet. The tube splits the air flow and improves the uniformity of burning of the solid fuel grain.
A device for controlling the rate of fuel burning, in the combustor, but not necessarily the uniformity thereof, is disclosed in U.S. Pat. No. 3,844,118 issued to Wilkinson on Aug. 28, 1973, wherein a valve moves to restrict the air inlet to the fuel grain in response to engine conditions, rather than ambient air conditions, to vary the total mass flow of air into the solid fuel combustor. Also, the ""118 patent discloses a device, which employs mechanical moving parts.
The tube-in-hole technique has proven effective in improving the distribution of air within the solid fuel combustor and uniformity of the fuel burn; however, these devices do not provide for continuously changing the distribution of air within the combustor in response to instantaneous changes in mass air flow or engine and missile parameters encountered in flight. Rather, the tube-in-hole is designed to give more uniform burning for a nominal flight condition, and over rich fuel conditions still present a problem. The tube-in-hole technique is described U.S. Pat. No. 4,628,688 issued to Keirsey on Dec. 16, 1986. U.S. Pat. No. 4,628,688 is incorporated by reference herein.
The valve arrangement set out, and other devices for throttling the air into the combustor, while effective in changing the total air flow through the combustor in a predetermined manner, and, thereby achieving a change in fuel burn rate, do not achieve the change in fuel burn rate by controlling the distribution of air over the fuel grain surface in response to instantaneous changes in air mass flow encountered. Rather, the valve arrangement responds only to engine variables, such as increasing fuel grain area or thrust, to throttle the air flow into the combustor, causing fuel production to increase or decrease, thereby achieving the desired result.
U.S. Pat. No. 4,628,688 discloses a solid fuel ramjet engine having means for increasing or decreasing the generation of fuel rich combustion gases as a function of changes in air mass flow rate into the solid fuel combustor, engine variables or missile flight conditions over its flight path. The fuel flow device includes a translating tube inserted into the air inlet of the fuel combustor for splitting the air flow into two portions, one directed along the fuel grain and the other directed through the center of the grain. A servomechanism continuously moves the tube longitudinally into and out of the fuel grain in response to instantaneous changes in the specific parameter being sensed, such as air mass flow as sensed by a sensor mounted in the air inlet. The position of the tube changes the airflow distribution and aerodynamic shear interaction of the air flow over the fuel grain surface which causes a change in the rate of fuel regression as a function of the instantaneous condition sensed.
The present invention is related to utilizing flow vortices for controlling heat transfer. The periodic shedding of vortices produced in highly sheared gas flows has been recognized as a source of substantial acoustic energy for many years. For example, experimental studies have demonstrated that vortex shedding from gas flow restrictors disposed in large, segmented, solid propellant rocket motors couples with the combustion chamber acoustics to generate substantial acoustic pressures. The maximum acoustic energies are produced when the vortex shedding frequency matches one of the acoustic resonances of the combustor. It has been demonstrated that locating the restrictors near a velocity antinode generated the maximum acoustic pressures in a solid propellant rocket motor, with a highly sheared flow occurring at the grain transition boundary in boost/sustain type tactical solid propellant rocket motors.
An apparatus and method for controlling pressure oscillations caused by vortex shedding is disclosed is in U.S. Pat. No. 4,760,695 issued to Brown, et al. on Aug. 2, 1988. The ""695 patent discloses an apparatus and method for controlling pressure oscillations caused by vortex shedding. Vortex shedding can lead to excessive thrust oscillations and motor vibrations, having a detrimental effect on performance. This is achieved by restricting the grain transition boundary or combustor inlet at the sudden expansion geometry, such that the gas flow separates upstream and produces a vena contract a downstream of the restriction, which combine to preclude the formation of acoustic pressure instabilities in the flowing gas stream. Such an inlet restriction also inhibits the feedback of acoustic pressure to the point of upstream gas flow separation, thereby preventing the formation of organized oscillations. The ""695 patent does not present a method or apparatus, which attempts to control pressure oscillations for the purpose of manipulating the thrust throttling capability of a ramjet engine.
The invention is related to solid fuel ramjet engines in missile systems, which do not possess thrust throttling capability, in their standard design. The lack of thrust throttling capability results in overspeeding. At higher speeds, the increasing air flow velocity over the fuel grain increases the regression rate of the fuel grain. As a result, the velocity is continually increased, which adversely affects the control and maneuverability and reduces the achievable range of the missile system. The present invention provides a device for controlling the regression rate in a solid fuel ramjet by exciting pressure oscillations in the combustor, rather than varying the air flow through the fuel tube. For example, to avoid overspeeding, pressure oscillations at controlled amplitude are exeited in the boost phase to achieve high thrust and the pressure oscillations are turned off during the cruise phase to achieve long range.
The regression rate in a solid fuel ramjet depends on the heat transfer from the boundary combustion zone into the solid fuel grain. To vary the heat transfer, and in turn the regression rate, pressure oscillations are excited using active, closed-loop control. The pressure oscillations are excited by introducing small disturbances into the shear layer. The disturbances are amplified in the shear layer and excite pressure oscillations. The disturbances are introduced at a certain phase relative to the pressure oscillations. This is accomplished by closed-loop control. By varying the phase, the amplitude of the pressure oscillations can also be varied, which in turn will vary the heat transfer, regression rate, and thrust level.
In a preferred embodiment of the present invention an actuator introduces small disturbances into the initial shear layer of the combustor of a solid fuel ramjet engine. The small disturbances are amplified in the shear layer and excite pressure oscillations. A sensor monitors the pressure oscillations and a controller determines the correct phase between the pressure oscillations and the actuator action.
One object of the present invention is to provide a method and apparatus for controlling the solid fuel regression rate of a solid fuel ramjet engine by varying the heat transfer from the combustion zone to the solid fuel grain using pressure oscillations.
Another object of the invention is to provide a method for on demand thrust throttling without the use of a device with mechanical moving parts.
Another object of the invention is to provide a method for on demand thrust throttling of a solid fuel ramjet engine by manipulation of the heat transfer across the solid fuel grain.
Another object of the invention is to provide a method for varying the heat transfer across the solid fuel grain of a solid fuel ramjet engine by varying the amplitude and frequency pressure oscillations in the combustor.
Another object of the invention is to provide an apparatus for varying the heat transfer across the solid fuel grain of a solid fuel ramjet engine by producing pressure oscillations.
A still further object of the invention is to provide a method for a more fuel efficient solid fuel ramjet engine.
A still further object of the present invention is to provide a ramjet engine, which can avoid overspeeding at high speeds, i.e. above Mach 4.